Method and apparatus for multi-carrier modulation (mcm) packet detection based on phase differences

ABSTRACT

A method is disclosed for detecting packet at a receiving system in a Multi-Carrier Modulation (MCM) system. The method starts with receiving a signal at the receiving system. Then a deviation value of the signal is computed, where the deviation value is computed at least partially based on phase differences between some number of carriers in the preamble. The deviation value is compared with a threshold to determine whether a packet has been detected from the received signal. In response to the determination that a packet has been detected, a symbol offset is computed optionally, where the symbol offset indicates a number of sample points from a beginning of a symbol.

RELATED APPLICATION

This application is related to co-pending U.S. patent application Ser.No. 13/838,211, entitled “Method and Apparatus for Phase-BasedMulti-Carrier Modulation (MCM) Packet Detection,” filed Mar. 15, 2013,which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to signal processing in a communication system.Specifically, it relates to packet detection in a Multi-CarrierModulation (MCM) system.

PRIOR ART AND RELATED ART

Multi-Carrier Modulation (MCM) is a method of encoding digital data onmultiple carrier frequencies. OFDM has been utilized in a wide varietyof communication systems, such as wireless or radio frequency (RF)systems, copper wire system, and power line communication (PLC) systems.In an MCM system, a number of carriers (sometimes referred to as carriersignals, the two terms are used interchangeably within thisspecification) are used to carry data on several parallel data streamsor channels. Each carrier is modulated with a modulation scheme (such asquadrature amplitude modulation or phase-shift keying) at a lower symbolrate, maintaining total data rates similar to conventionalsingle-carrier modulation schemes in the same bandwidth. When thecarriers are orthogonal to each other at a MCM system, the MCM system isgenerally referred to as an Orthogonal Frequency-Division Multiplexing(OFDM) system. Because OFDM systems are the most popular forms of MCMsystems so far, all MCM systems with non-orthogonal carriers are oftenreferred to as non-OFDM MCM systems or simply non-OFDM systems.

In designing an MCM receiving system, finding a cost-effective carrierdetection scheme is often a challenge, particularly when the MCMreceiving system is required to be low cost or low power thus cannotimplement a powerful processor. Thus, correlation-based carrierdetection known in the art may not be viable in this kind of MCMreceiving systems and new ways of carrier and packet detection isneeded.

SUMMARY OF THE INVENTION

A method is disclosed for detecting packet at a receiving system in aMulti-Carrier Modulation (MCM) system. The method starts with receivinga signal at the receiving system. Then a deviation value of the signalis computed, where the deviation value is computed at least partiallybased on phase differences between some number of carriers in thepreamble. The deviation value is compared with a threshold to determinewhether a packet has been detected from the received signal. In responseto the determination that a packet has been detected, a symbol offset iscomputed optionally, where the symbol offset indicates a number ofsample points from a beginning of a symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a data frame structure used for data transmission ina Multi-Carrier Modulation (MCM) system.

FIG. 2 illustrates a number of unmodulated carriers in a Multi-CarrierModulation (MCM) system.

FIG. 3 illustrates a snapshot of an OFDM packet in an orthogonalfrequency-division multiplexing (OFDM) system.

FIG. 4 illustrates a close-up frequency domain view of the preamble ofan OFDM packet in an orthogonal frequency-division multiplexing (OFDM)system.

FIG. 5 illustrates a method of MCM packet detection according to oneembodiment of the invention.

FIG. 6 illustrates a process of MCM packet detection according to oneembodiment of the invention.

FIG. 7 illustrates a snapshot of phase differences of carriers of asingle symbol of a live OFDM receiving system according to oneembodiment of the invention.

FIG. 8 illustrates a snapshot of phase difference measurements ofcarriers of a single symbol for two different offset cases of a liveOFDM receiving system according to one embodiment of the invention.

FIG. 9 illustrates an apparatus implementing the packet detection andsynchronization methods according to an embodiment of the invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. It will beappreciated, however, by one skilled in the art that the invention maybe practiced without such specific details. Those of ordinary skill inthe art, with the included descriptions, will be able to implementappropriate functionality without undue experimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.“Coupled” is used to indicate that two or more elements, which may ormay not be in direct physical or electrical contact with each other,co-operate or interact with each other. “Connected” is used to indicatethe establishment of communication between two or more elements that arecoupled with each other.

FIG. 1 illustrates a data frame structure used for data transmission ina Multi-Carrier Modulation (MCM) system. Packet 100 includes preamble102 with a number of P symbols and 1½ M symbols. In one embodiment,there are 8 P symbols in preamble 102. P symbols may be used for symbolsynchronization, channel estimation, initial phase reference estimation,and automatic gain control (AGC). For M symbols, two types of symbol maybe used. One is the M1 in which all the carriers may be π phase shiftedand the other one is M2 in which all the carriers may be π/2 phaseshifted. At the receiver, the phase distance between symbol P and symbolM waveforms may be used for packet frame synchronization purpose.

The preamble consists of a set of unmodulated carriers (or carrierswithout modulation, the two terms are used interchangeably within thespecification) transmitted within a duration of multiple symbol times.The frequencies of these carriers generally are multiples of some basefrequency and each carrier may contain a different initial phase.Preamble 102 is transmitted before data symbols 104, which contains anumber of symbols. Data symbols 104 may not use the same set of carriersas the preamble 102. In addition, data symbols 104 use modulatedcarriers though modulation schemes such as phase-shifting keying (PSK).

Note that data symbols 104 are generally modulated using square wavephase modulation in an OFDM system. In a non-OFDM MCM system, datasymbols 104 uses other phase modulation such as Nyquist shaped phasemodulation. For packet detection/synchronization, the focus is on thepreamble—detecting its carriers and their phases while the differencesof data symbol modulation schemes between an OFDM and non-OFDM MCMsystem are of little concern. Thus, while embodiments of the inventionsherein are often disclosed using OFDM systems only as examples, theembodiments of the inventions may be used in other non-OFDM MCM systemsas well.

Packet 100 is sent from an MCM transmitting system, going through atransmission channel (e.g., wireless/RF channel, copper wire, or a powerline), and arrived at an MCM receiving system. In designing an MCMreceiving system, the goal should be to make the data decoding function,not the packet detection/synchronization function, be the limitingfactor on whether or not a packet is successfully decoded. This shouldbe achievable since packets can be designed so that there is moreredundancy in the preamble section than the data symbol section, thusdata errors should limit packet reception. Yet, an MCM receiving systemusing correlation-based carrier detection techniques known in the artmay not be able to achieve the goal due to several drawbacks.

A correlation-based carrier detection technique tends to becomputationally intensive. A correlation requires N²multiple-accumulates for an N-point symbol. For an MCM receiving systemrequired to be low cost or low power (e.g., a power line modem), adigital signal processing (DSP) processor with less computing power isdesirable. Yet a DSP processor with less computing power may not be ableto perform computationally expensive algorithm like N²multiple-accumulates.

In addition, the output value of a correlation-based detection isgenerally a function of packet amplitude. Packet amplitude sometimesvaries over an extremely wide range and is unknown at the time thealgorithm is in play. For example, in a power line communication (PLC)system, the packet amplitude varies over a range of 80 dB. In addition,the output value also varies with noise level, which is unknown becauseit varies over time. Proper detection involves knowing what level ofcorrelation out to expect which requires normalization or estimates ofpacket levels. Techniques are known in the art to estimate the packetlevel, but they are not desirable and poor estimate causes performanceissues.

Furthermore, in some systems, correlation values in the presence ofnoises are poor. For example, in a PLC system, the correlation valuesare poor for certain types of noises commonly found on the power line(e.g., large harmonically rich tones). In these cases, the packetwaveform can be dominated by a few impairment harmonics such that verylow correlations result.

With the drawbacks discussed above, correlation-based carrier detectionis not suitable for PLC systems or other systems sharing thecharacteristics of PLC systems (e.g., requiring low-power or low-costreceiving systems, having wide range or unknown amplitude packetamplitude, and/or noise correction values being low). Thus, a newapproach of preamble carrier detection and symbol offset determinationfor packet detection is needed.

As shown in FIG. 1, MCM packets are composed of many symbols, each ofwhich contains a multiplicity of carriers. The carrier frequencieschosen are usually all multiples of a single frequency f₀ which ischosen to be the inverse of the computed symbol time T_(s). A judiciouschoice combined with picking the input sample rate of a receiving systemto be a binary multiple N of T_(s) allows the demodulation of thesymbols to be accomplished with the use of a discrete Fourier Transform(DFT). The DFT is commonly implemented as a fast Fourier Transform(FFT), although other DFT methods may be utilized. Note that the actualsymbol time TA can be extended to be longer than T_(s) by adding acyclic prefix to allow for dispersion in the channel, but the transformis computed using a subset of N points.

When a binary phase-shift keying (BPSK) modulation is used, data isencoded by square wave phase modulating each carrier with a peak-to-peakdeviation of π radians around some chosen reference phase θ_(k). Thusthe nth symbol could be represented mathematically as function of time tby

$S_{n} = {\sum\limits_{k = c_{0}}^{c_{0} + n_{c} - 1}\; {A_{k}^{j{({{2\; \pi \; f_{0}k\; t} + \theta_{k} + {\pi \; d_{k,n}}})}}}}$

where c₀ is the number of the first of n_(c) carriers and d_(k,n) is thebinary data for the k^(th) carrier of the n^(th) symbol.

At the receiving system, decoding of these symbols requires that a validpacket has been detected and that the symbol boundaries have beendetermined. As illustrated in FIG. 1, a preamble is usually pre-pendedto the data section of the packet to facilitate this packet detectionand synchronization. The number of constant non-modulated P-symbols may(although not must) use a same frequency carrier set as the data portionof the packet. This has the advantage of having a very similar frequencyenergy distribution as the data section of the packet and being uniqueenough that it can be distinguished from other non-packet energy (e.g.noise or interference) that can exist on the channel So the n^(th)P-symbol can be represented by

$P_{n} = {\sum\limits_{k = c_{0}}^{c_{0} + n_{c} - 1}{A_{k}^{j{({{2\; \pi \; f_{0}t\; k} + \theta_{k}})}}}}$

which is essentially a data symbol except all the data values have beenset to zeros.

FIG. 2 illustrates a number of unmodulated carriers in a Multi-CarrierModulation (MCM) system. In FIG. 2, 36 carriers (F0-F35) are utilizedfor preamble symbols. The 36 unmodulated carriers are evenly spaced intofrequency bands and they may have the same power spectral density.

The number of carriers for the preamble of an MCM system is oftenaffected by regulations in various countries and standardization bodies.For example, for power line communication (PLC) system, only certainfrequency bands of the power line are allowed to be used forcommunication. Various standardization bodies are involved inregulations of frequency bands: Federal Communication Commission (FCC)in the United States, Association of Radio Industries and Businesses(ARIB) in Japan, and European Committee for ElectrotechnicalStandardization (CENELEC) in Europe. Table 1 illustrates variousfrequency bands for PLC system per standardization bodies.

TABLE 1 FCC, ARIB, and CENELEC Bands Frequency Low (KHz) Frequency High(KHz) FCC 10 480 ARIB 10 450 CENELEC A 9 95 CENELEC B 95 125 CENELEC C125 140 CENELEC B, C 95 140

Under these regulations, the number of carriers allowable in eachfrequency band is a constraint. In one embodiment of an OFDM system, itis assumed that the maximum spectral content of signals is 480 KHz, thesampling frequency at the transmitter and receiver may be selected to be1.2 MHz that is about 240 KHz above the Nyquist rate to provide asufficient margin. A fast Fourier Transform may be implemented for DFTand 256 frequency bins may be selected, which results in a frequencyresolution for the OFDM carriers equal to 4.6875 KHz (Fs/N). Table 2illustrates the number of allowable carriers for various frequency bandsper standardization bodies in one embodiment under the assumption. Foran OFDM receiving system (e.g., a PLC modem) regulated under FCC orARIB, 36 carriers illustrated in FIG. 2 can be deployed freely. Yet, thesame OFDM receiving system may not be deployed in Europe at CENELEC Band/or C bands. In this specification, a small number of carriers isdefined as no higher than 18. Thus, small carrier sets are sets ofcarriers with no more than 18 carriers. While the embodiments of thisinvention may be utilized in an OFDM system with a small number ofcarrier sets in some scenarios, the focus of this specification is onthe OFDM system with a larger carrier sets. Note a non-OFDM MCM systemhas similar constraints on the number of carriers allowed in frequencybands and embodiments of this invention may be utilized in the non-OFDMMCM system as well.

TABLE 2 Number of Carriers for Various Bands Number Last Carrier ofCarriers First Carrier (KHz) (KHz) FCC 100 14.063 478.125 ARIB 9314.0625 445.3125 CENELEC A 19 9.375 93.75 CENELEC B 6 98.4375 121.875CENELEC C 3 126.5625 135.9375 CENELEC B, C 9 98.4375 135.9375

FIG. 3 illustrates a snapshot of an OFDM packet in an orthogonalfrequency-division multiplexing (OFDM) system. The packet has preamble302 consisting of 36 carriers. Each symbol of preamble 302 hasconsistent waveforms. The data symbols 304 vary widely in theirwaveforms.

FIG. 4 illustrates a close-up view of the preamble of an OFDM packet inan orthogonal frequency-division multiplexing (OFDM) system. A fastFourier Transform (FFT) has been performed on the preamble of an OFDMpacket illustrated in FIG. 3 and the close-up view of FIG. 4 is the FFTof the preamble. The first carrier 402 is the first of a series of 36carriers, and first delta 404 illustrates a frequency difference betweentwo adjacent carriers.

The new approach of carrier detection starts from an observation of thecarriers and their phases in an MCM system. The phase difference betweenadjacent carriers, after subtracting the initial phase differenceoffsets of the ideal waveforms, is (1) the same for every carrier pair,(2) proportional to the offset, and (3) independent of amplitude. Thussuitably compensated phase differences of the carriers of preamble of apacket are the same. One may detect the carriers of preamble of thepacket based on this equality of differences and the synchronization isachieved due to the proportionality of the calculation's output value tosymbol offset. The equality of differences is true for the packetscomposed of exactly the set of carriers, but not for noise or othercommon impairment.

When impairments consisting of multiple large in-band harmonics areadded to the spectrum of preamble carriers, up to several of the phasedifferences will be affected. Slightly larger than the packet carrieramplitude, the impairments harmonics will dominate only those few phasemeasurements which are near the carrier bins, and further amplitudeincreases of the impairment will have little or no effect on output ofthis approach. That is, the effect on the equality of differencescomputation (unlike a correlation-based calculation) becomes relativelyindependent of the impairments amplitude.

A large number of carriers may be deployed for preambles of packets inmany MCM systems (see table 2 herein above for an OFDM system regulatedfor PLC). With a large number of carriers (e.g., 18 or more carriers),the contamination of only a relatively few phase measurements (e.g.,caused by the impairments harmonics) results in the overall computationbeing only slightly degraded. Thus, for an MCM system with a largenumber of carriers, the approach of examining equality of phasedifferences is robust in the presence of these types of impairments. Foran MCM system with a smaller number of carriers (e.g., 18 or lesscarriers), the contamination of large in-band harmonics may makeutilizing embodiments of this invention more challenging, and analternative packet detection approach has been disclosed in theco-pending U.S. patent application with title and authorship disclosedin the paragraph 0001 of this specification. Note embodiments of theinvention can still be utilized for packet detection in an MCM systemwith a smaller number of carriers where multiple large in-band harmonicsis not a dominating concern.

In examining equality of phase difference, one may start with perfectlyaligned symbols. When perfectly aligned with a symbol boundary, theoutput of the discrete Fourier Transform (DFT) of an aligned P-symbolwill yield k complex values A_(k)e^(jθk), representing the referencephases for each carrier used. Before a packet is detected, the alignmentof the points used in the computation of the DFT (commonly implementedwith FFT) is necessarily arbitrary since the symbol boundaries are notknown yet by the receiving system. When offset from the symbol boundaryby r points the Discrete Fourier Transform output will yield a set ofcomplex values where the value for the k^(th) carrier is

γ_(k) =A _(k) e ^(j(2πf) ⁰ ^(kr+θ) ^(k) ⁾

It is advantageous to be able to detect a packet by performing acomputation only once every symbol time versus once every sample timeand for that computation to yield the symbol offset and come up with apacket indication independent of the offset r. Ideally the packetdetection computation yields a single value that is approximatelyproportional to the probability that the computation was performed onsamples taken from a valid packet preamble. This single packet detectionprobability value can then compared to a threshold value to signaldetection at the desired confidence level.

FIG. 5 illustrates a method of MCM packet detection according to oneembodiment of the invention. Method 500 may be implemented in an MCMreceiving system such as a PLC modem or any other system that sharescharacteristics of a PLC system. In an MCM system, data traffic isformatted as MCM packets to transmit through a transmission channel. Thetransmission channel may be a wireless/RF channel, copper wire, a powerline, or others. An MCM packet includes a preamble, which consists ofcarriers without modulation, and each carrier contains an initial phase.

Method 500 starts at reference 502 with receiving a signal at the MCMreceiving system. The signal may be obtained from sampling thetransmission channel. Then the MCM receiving system computes a deviationvalue of the signal at reference 504. The deviation value is computed atleast partially based on phase differences between some number ofcarriers of the preamble. As discussed in more detail herein below,there are various embodiments to obtain the set of phase differencesdepending on selecting of carriers for the computation, initial phasedifferences of the carriers, means to arrive at the deviation value, andother variables.

After the deviation value is derived, it is compared with a threshold atreference 506 to determine whether a packet has been detected from thereceived signal. Afterward, method 500 may continue at reference 508 anda symbol offset is computed in response to the determination that apacket has been detected. The computed symbol offset indicates a numberof sample points from a beginning of a symbol. With a determination ofpacket detection and symbol offset, the MCM receiving system can decodethe preamble and thus decode the data carried in the MCM packet. Notemethod 500 can be implemented in numerous ways depending on factors suchas the characteristics of the MCM system, hardware/software constraintsof system design and preference of embodiments.

FIG. 6 illustrates a process of MCM packet detection according to oneembodiment of the invention. Method 600 may be implemented in an MCMreceiving system such as a PLC modem or any other system that sharescharacteristics of a PLC system. Method 600 discloses a step-by-stepprocess to implement method 500 for illustration purpose but it is notmeant to be the only way implementing method 500.

Method 600 starts at reference 602 with performing a discrete Fouriertransform (DFT) on a received signal at the MCM receiving system. Thesignal may be obtained by sampling the transmission channel at a ratef_(s)=N/T_(s) where T_(s) is the computed (non-extended) symbol time.The sampling frequency f_(s) and the binary integer N are chosen suchthat the resulting resolution of a Discrete Fourier Transform (oftenimplemented with a fast Fourier Transform (FFT)) of N samples sampled atf_(s) equals the MCM carrier spacing. When an FFT is utilized, the MCMreceiving system collects N consecutive samples and performs an N-pointFFT on the sample set. The result contains the subset of n_(e) complexvalues

γ_(k) =A _(k) e ^(j(2πf) ⁰ ^(kr+θ) ^(k) ⁾

for k=c₀ to c₀+n_(c)−1, one for each carrier of the preamble.

Then at reference 604, a set of angles for the carriers is computedbased on the result of the DFT. The angle

α_(k)=2πf ₀ kr+θ _(k)

of each γ_(k) is obtained by taking the arctangent of the ratio of thereal and imaginary components of the FFT output for those n_(c)frequency bins that are MCM carriers. Note that this angle should bemodulo 2π.

Onward to reference 606, a set of angular differences of the carriers iscomputed. The set of angular differences may be angular differencesbetween adjacent frequency bins that are carriers for the preamble. Withn_(c) frequency bins, there are n_(c)−1 angular differences expressed as

α_(k+1)−α_(k)

In another embodiment, the angular differences may be sampled betweennon-adjacent frequency bins (e.g., every other frequency bins). In yetanother embodiment, not all the n_(c) frequency bins are utilized inobtaining angular differences.

Then optionally at reference 608, a set of offsets is subtracted fromangular differences obtained in reference 606. Reference 608 is notneeded when all the carriers for the preamble have the same initialphase. When the carriers have different initial phases, the angulardifference is adjusted by subtracting the differences of the initialphases in one embodiment, thus the MCM receiving system obtains a set ofadjusted angular differences d_(m) through computation such as:

d _(m)=(α_(k+1)−α_(k))−(θ_(k+1)−θ_(k))

where m=0 to n_(c)−1 and k=c₀ to c₀+n_(c)−1.

Onward to reference 610, the MCM receiving system computes a mean of theset of angular differences, or a mean of the set of adjusted angulardifferences when the initial phases of the carriers are different. Themean D is defined in one embodiment as the angle formed from the meansof the real and imaginary components of d_(m):

$D = {a\; \tan \; 2^{- 1}\left( \frac{\sum\limits_{m = 0}^{n_{c} - 2}\; {\cos \; \left( d_{m} \right)}}{\sum\limits_{m = 0}^{n_{c} - 2}{\sin \; \left( d_{m} \right)}} \right)}$

where atan 2 gives the four quadrant arctangent angle.

Then at reference 612, a deviation value of the angular differences (oradjusted angular differences when the initial phases of the carriers aredifferent) from the mean is computed. The deviation value, designated as∇, can be computed in a variety of ways. In one embodiment, it iscomputed by calculating the sum of the absolute values of the differencebetween d_(m) and D, i.e.,

$\nabla{= {\sum\limits_{m = 0}^{n_{c} - 2}{{abs}\; \left( {d_{m} - D} \right)}}}$

In another embodiment, the deviation value ∇ may be computed bycalculating differently. For example, the deviation value may becomputed by calculating a root mean square (RMS) of the differencebetween d_(m) and D.

The steps of references 602 to 612 may be reiterated several times forseveral symbol times in a row as indicated in reference 614. An operatorof the MCM system may predetermine the proper number of iterations for agiven implementation in one embodiment. In one embodiment, the number ofiterations is configurable by software, firmware, hardware, or acombination thereof. When the number of iterations is set to one, thesystem does not reiterate and the step of references 602 and 612 will beperformed only once. When the number of iterations is set to more thanone, the system determines a way to combine the deviation values ∇ fromdifferent iterations. A variety of combination method may beimplemented. For example, a running average of the last M calculationsof ∇ is computed, and M=4 is used in one embodiment. In anotherembodiment, a form of weighted average of the ∇ is used to derive asingle average deviation value ∇ from a number of deviation values.

Once the deviation value is computed, it is compared to a thresholdvalue at reference 616. If the deviation value is below the thresholdvalue, the MCM receiving system determines that a packet has beendetected. The closer to zero the deviation value ∇ is, the higher theprobability that there is a valid packet present. The threshold value isstored in the MCM receiving system and it may be predetermined by thesystem in one embodiment. In another embodiment, the threshold value isconfigurable and the configuration is through software, firmware,hardware, or a combination thereof.

Once it is determined that a packet has been detected. Optionally method600 continues at reference 618 to compute a symbol offset. The symboloffset indicates a number of sample points from a beginning of a symbol.The symbol offset, designated as O_(s), is computed through:

$O_{s} = \frac{D*N}{2\; \pi}$

In an ideal MCM system where impairments due to channel noises and otherfactors are nonexistent, the deviation value ∇ during the preamblesymbol period should be zero. The deviation value ∇ is relativelyinsensitive to the amplitude level of MCM packets. Thus comparingdeviation value ∇ (or an average deviation value ∇ where multipleiterations are performed) to a fixed threshold can be used to determinepacket detection over a wide range of amplitude and noise conditions inthe channel. This overcomes one of the major drawbacks of using acorrelation-based algorithm as discussed herein above where thecomparison threshold is a function of packet amplitude and the packetamplitude is generally unknown.

Furthermore, embodiments of this invention are more robust than thedecoding the data portion of the packet which means the limiting factorfor reliably decoding an MCM packet will be the data decoding and notthe packet detection. The reason packet detection is more reliable isbecause both packet detection and data decoding rely on the accuracy ofdetermining the received angles for each carrier, and yet the redundancyis generally greater for packet detection so it can tolerate greaterphase errors and still function. The errors are randomly distributed andtend to cancel out each other. To decode a single bit of information,whether a valid packet is present or not, there are n_(c)*M (e.g.36*4=144 for one MCM system) phase angle measurements used. This is amuch higher ratio of redundancy than the data section where the ratiotypically ranges from 2-10. Thus, the process illustrated in FIG. 6results in data decoding function be the limiting factor on whether ornot a packet is successfully decoded, not packet detection based onpreamble carrier detection.

The reasoning behind the embodiments of the invention as illustrated inFIGS. 5 and 6 can be explained mathematically. For clarity andsimplicity of discussion, noise and other impairments are left out. Itis assumed that an MCM preamble consists of multiple identical preambleP symbols

$P_{n} = {\sum\limits_{k = c_{0}}^{c_{0} + n_{c} - 1}\; {A_{k}^{j{({{2\; \pi \; f_{0}t\; k} + \theta_{k}})}}}}$

where c₀ is the number of the first carrier in the MCM packet and n_(c)is the number of carriers, and A_(k) is the amplitude of the k^(th)carrier. The frequency f₀=f_(s)/N is the fundamental frequency which isequal to the frequency spacing of the MCM packet.

After sampling at f_(s), P_(i) is obtained as:

$P_{i} = {\sum\limits_{k = c_{0}}^{c_{0} + n_{c} - 1}{A_{k}^{j{({\frac{2\; \pi \; k\; }{N} + \theta_{k}})}}}}$

since f₀=f_(s)/N and t=i/f_(s) where i is a sample number. After theDiscrete Fourier Transform of a set of N points r offset from a P symbolboundary, the subset of n_(c) relevant Fourier Transform complex outputvalues are

$\gamma_{k} = \left\{ {{A_{k}^{j{({\frac{2\; \pi \; c_{0}r}{N} + \theta_{k}})}}},{A_{k + 1}^{j{({\frac{2\; {\pi {({c_{0} + 1})}}r}{N} + \theta_{k + 1}})}}},\cdots \mspace{14mu},{A_{k + n_{c} - 1}^{{j{({\frac{2\; {\pi {({c_{0} + n_{c} - 1})}}r}{N} + \theta_{k + n_{c} - 1}})}}\}}}} \right.$

Taking the 4-quadrant atan 2⁻¹ of the ratio of the imaginary and realcomponents of this subset yields:

$\alpha_{k} = \left\{ {{\frac{2\; \pi \; c_{0}r}{N} + \theta_{k}},{\frac{2\; {\pi \left( {c_{0} + 1} \right)}r}{N} + \theta_{k + 1}},\cdots \mspace{14mu},{\frac{2\; {\pi \left( {c_{0} + n_{c} - 1} \right)}r}{N} + \theta_{k + n_{c} - 1}}} \right\}$

Now taking (n_(c)−1) modulo 2π differences between these angles yieldsthe set of values

$\left\{ {{\frac{2\; {\pi \left( {c_{0} + 1} \right)}r}{N} + \theta_{k + 1} - \frac{2\; \pi \; c_{0}r}{N} + \theta_{k}},{\frac{2\; {\pi \left( {c_{0} + 2} \right)}r}{N} + \theta_{k + 1} - \frac{2\; {\pi \left( {c_{0} + 1} \right)}r}{N} + \theta_{k}},\cdots \mspace{14mu},} \right\}$

Next subtract the ideal set of delta angles (θ_(k+1)−θ_(k)) of a validpreamble from the differences above and reduce the expression to yieldthe n_(c) set of values

$d_{m} = \left\{ {\frac{2\; \pi \; r}{N},\frac{2\; \pi \; r}{N},{\cdots \mspace{14mu} \frac{2\; \pi \; r}{N}}} \right\}$

which is a set of identical values which are function of the sampleoffset r.

Since there was no noise accounted for in this simplified discussion,the mean value is:

$D = \frac{2\; \pi \; r}{N}$

Note that the mean D must be taken modulo 2π and care must be taken whenaveraging angles that wrap.

Since all d_(m) values are equal (or close to equal when some impairmentis included), it is derived:

∇=Σ_(m=0) ^(n) ^(c) ⁻² abs(d _(m) −D)=0

a result that is independent of both the sample offset r and packetamplitude A.

Furthermore, symbol synchronization can be implemented using theinformation that the symbol offset in samples is:

$r = \frac{D*N}{2\; \pi}$

FIG. 7 illustrates computation results for a carrier detection of an MCMreceiving system according to one embodiment of the invention. Thepreamble of the MCM receiving system consists of 36 carriers, F0-F35.When angular differences between the carriers subtracting the initialphase differences between the carriers is computed, they are alignedclose to each other in values. Deviation value 702 is calculated throughways discussed herein above in connection with references 602-614.

FIG. 8 illustrates snapshots of computation results for carrierdetections of a live OFDM receiving system according to one embodimentof the invention. The snapshots are two examples of the results ofpreamble carrier detection during a valid preamble. There are 35individual (adjusted) deviation values (sometimes referred to ascompensated carrier phase differences) d_(m) plotted as asterisks, onevalue for each adjacent pair of carriers. The mean value of the set of35 d_(m) values associated with one calculation is represented by thedotted horizontal lines, but is scaled by N/2π. The (adjusted) deviationvalues are approximated by the spread of the dots around the mean valueD and are shown with brackets.

The deviation values approximated by the brackets are compared to athreshold to determine whether a packet has been detected. Noteadjustment of deviation values results from a means to average thedeviation values as discussed hereinabove. Reference 802 illustrates thesymbol offset of 20 samples from the symbol boundary and reference 804illustrates the symbol offset of 30 samples. Note that while FIG. 8 usesa living OFDM receiving system to illustrate packet detection accordingembodiments of the inventions, a non-OFDM MCM receiving system canutilize packet detection utilizing embodiments of the invention as well.

FIG. 9 illustrates an apparatus implementing the packet detection andsynchronization methods according to an embodiment of the invention.Apparatus 900 is implemented as an MCM receiving system. The MCMreceiving system may be a part of an MCM system utilizing a transmissionchannel of a power line, a radio frequency channel, an optical fiber, ora copper line, depending on implementation. When the transmissionchannel is a power line, the MCM system complies with G3-Power LineCommunication physical layer specification in one embodiment. In the MCMsystem, traffic is modulated as packets and transmitted packets includepreamble for packet detection. The preamble for each packet consists ofa number of carriers without modulation (i.e., unmodulated carrier), andeach carrier contains an initial phase.

Apparatus 900 contains signal interface logic 902, phase deviationprocessor 920, symbol offset computing logic 944, and setting database912. These modules are communicatively coupled via interconnect 939,which may be a bus connection in one embodiment. Note apparatus 900contains other modules and logic not shown as they are not essential toembodiments of the invention. The various logics may be implemented as asingle unit, or multiple units can combine two or more units withinapparatus 900. Not all embodiments of the invention contain all logicsdisclose herein and some logics are not utilized in some embodiments andthey may not be implemented these embodiments. Also, deviationcomputation processor 920 can be general purpose or special purposeprocessors. The individual logics can contain their dedicated networkprocess units (NPUs) or they can share NPUs among multiple logics.

In one embodiment, phase deviation processor 920 comprises discreteFourier transfer (DFT) processor 922, angular difference logic 924, anddeviation computation logic 926. In one embodiment, DFT processor 922 isimplemented with a fast Fourier transform (FFT). Angle difference logic924 is configured to compute a set of angular differences from a set ofcarriers based on the result of DFT process 922. The set of angulardifferences is angular differences between adjacent frequency bins thatare carriers for a preamble of the signal in one embodiment. In oneembodiment where the carriers have different initial phases, the set ofangular differences are adjusted by subtracting the differences of theinitial phases of the carriers. Angular difference logic 924 is furtherconfigured to compute a mean of the set of angular differences. The meancan be computed in a variety of ways known in the art. The results ofthe computations are then forwarded to deviation computation logic 926,which is configured to derive a deviation value from the set of angulardifferences. As discussed herein above, the deviation value can bederived in a variety of ways known in the art. Then the deviation valueis used to compare with a threshold value stored in setting database 912to determine if a packet has been detected. If the phase variance sum islower than the threshold value, it's determined that a packet has beendetected.

Once a packet has been detected, symbol offset computing logic 944 isconfigured to determine the symbol offset to synchronize with thepacket. The symbol offset indicates a number of sample points from thebeginning of a symbol. The symbol offset is calculated though forming aweighted average of phase differences between some number of carriers inthe preamble. For example, the phase differences can be a set of phasedifferences between adjacent carriers in the preamble.

The operations of the flow diagram have been described with reference tothe exemplary embodiment of FIG. 9. However, it should be understoodthat the operations of flow diagrams can be performed by embodiments ofthe invention other than those discussed with reference to FIGS. 5 and6, and the embodiments discussed with reference to FIG. 9 can performoperations different than those discussed with reference to the flowdiagrams of FIGS. 5 and 6.

Different embodiments of the invention may be implemented usingdifferent combinations of software, firmware, and/or hardware. Thus, thetechniques shown in the figures can be implemented using code and datastored and executed on one or more electronic devices (e.g., an endsystem, a network device). Such electronic devices store and communicate(internally and/or with other electronic devices over a network) codeand data using computer-readable media, such as non-transitorycomputer-readable storage media (e.g., magnetic disks; optical disks;random access memory; read only memory; flash memory devices;phase-change memory) and transitory computer-readable transmission media(e.g., electrical, optical, acoustical or other form of propagatedsignals—such as carrier waves, infrared signals, digital signals). Inaddition, such electronic devices typically include a set of one or moreprocessors coupled to one or more other components, such as one or morestorage devices (non-transitory machine-readable storage media), userinput/output devices (e.g., a keyboard, a touchscreen, and/or adisplay), and network connections. The coupling of the set of processorsand other components is typically through one or more busses and bridges(also termed as bus controllers). Thus, the storage device of a givenelectronic device typically stores code and/or data for execution on theset of one or more processors of that electronic device.

While the flow diagrams in the figures herein above show a particularorder of operations performed by certain embodiments of the invention,it should be understood that such order is exemplary (e.g., alternativeembodiments may perform the operations in a different order, combinecertain operations, overlap certain operations, etc.).

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, can be practiced with modificationand alteration within the spirit and scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting.

What is claimed is:
 1. A machine-implemented method of detecting packetsat a receiving system in a Multi-Carrier Modulation (MCM) system,wherein packets are transmitted through the MCM system, wherein eachtransmitted packet includes a preamble for packet detection, wherein thepreamble for each packet consists of a number of carriers withoutmodulation, wherein each carrier contains an initial phase, and whereinthe number of carriers are transmitted in a plurality of a symbolduration, the method comprising: receiving a signal at the receivingsystem; computing a deviation value of the signal, wherein the deviationvalue is computed at least partially based on phase differences betweensome number of carriers in the preamble; and comparing the deviationvalue with a threshold to determine whether a packet has been detectedfrom the received signal.
 2. The machine-implemented method of claim 1,wherein the phase differences are between pairs of immediately adjacentcarriers.
 3. The machine-implemented method of claim 1, whereinobtaining the phase differences between the some number of carriers inthe preamble includes removing initial phase differences of the somenumber of carriers.
 4. The machine-implemented method of claim 1,wherein the deviation value computation includes summing up the phasedifferences between the some number of carriers in the preamble.
 5. Themachine-implemented method of claim 4, wherein the summing up the phasedifferences between the some number of carriers in the preamble includesforming a weighted average of the phase differences.
 6. Themachine-implemented method of claim 5, wherein the deviation valuecomputation includes computing a deviation from the weighted average ofthe phase differences.
 7. The machine-implemented method of claim 1,wherein the number of carriers is no less than
 18. 8. Themachine-implemented method of claim 1, wherein the MCM system is a powerline communication (PLC) system.
 9. The machine-implemented method ofclaim 8, wherein the PLC system complies with G3-Power LineCommunication (PLC) physical layer specification.
 10. Themachine-implemented method of claim 1, further comprising computing asymbol offset in response to the determination that a packet has beendetected, wherein the symbol offset indicates a number of sample pointsfrom a beginning of a symbol.
 11. The machine-implemented method ofclaim 10, wherein the computing the symbol offset includes forming aweighted average of the phase differences between the some number ofcarriers in the preamble.
 12. The machine-implemented method of claim 1,wherein the MCM system is an orthogonal frequency-division multiplexing(OFDM) system.
 13. An apparatus implemented as a receiving system in aMulti-Carrier Modulation (MCM) system, wherein traffic is modulated aspackets transmitted through the MCM system, wherein each transmittedpacket includes a preamble for packet detection, wherein the preamblefor each packet consists of a number of carriers without modulation, andwherein each carrier contains an initial phase, the apparatuscomprising: a signal interface logic configured to receive signals; aphase deviation processor configured to compute a deviation value of thesignal, wherein the deviation value is computed at least partially basedon phase differences between some number of carriers in the preamble,and the phase deviation processor further configured to compare thedeviation value obtained with a threshold to determine whether a packethas been detected from the received signal; and a setting databaseconfigured to store the threshold.
 14. The apparatus of claim 13,wherein deviation computation processor comprises: a delta computationlogic configured to remove initial phase differences of the some numberof carriers.
 15. The apparatus of claim 13, wherein the deviation valuecomputation includes summing up the phase differences between the somenumber of carriers in the preamble.
 16. The apparatus of claim 15,wherein the summing up the phase differences between the some number ofcarriers in the preamble includes forming a weighted average of thephase differences.
 17. The apparatus of claim 16, wherein the deviationvalue computation includes computing a deviation from the weightedaverage of the phase differences.
 18. The apparatus of claim 13, whereinthe number of carriers is larger than
 18. 19. The apparatus of claim 13,wherein the MCM system is a power line communication (PLC) system. 20.The apparatus of claim 19, wherein the PLC system complies with G3-PowerLine Communication (PLC) physical layer specification.
 21. The apparatusof claim 13, further comprising a symbol offset computing logicconfigured to compute a sample offset in response to the determinationthat a packet has been detected, wherein the symbol offset indicates anumber of sample points from a beginning of a symbol.
 22. The apparatusof claim 13, wherein the MCM system is an orthogonal frequency-divisionmultiplexing (OFDM) system.